The invention refers to a piston compressor for the oil-free compression of gases, having at least one cylinder and a piston guided in it and provided with at least one split, so-called trapped piston ring which is accommodated in a ring groove on the piston. Its circumferential face remote from the inner face of the cylinder defines a space in the ring groove which is acted upon by the pressure of the gas in the compression space in the cylinder, and which, starting from the circumferential face, has two radial surfaces which are parallel with one another. A face there of next to the compression space extends over part of the radial width of the ring, the piston ring being guided by its two parallel surfaces to slide against corresponding faces of the ring groove.
Such a piston compressor of that kind with trapped piston rings is known from Swiss patent 482 953. Each trapped piston ring has a collar projecting in the axial direction, either only from the side next the compression space so that in cross-section the piston ring has an angular profile, or from both sides of the ring in the axial direction so that a T-shaped cross-section results. As seen in the radial direction the projecting collar is behind a shoulder projecting axially from the annular groove, this shoulder forming a stop for the piston ring when it widens under the pressure from the gas acting on its inner circumferential face. Because of this expansion limitation the piston rings are called trapped rings.
In operation of the piston compressor three phases result for these piston rings:
1. The piston ring is not yet lying with its collar against the shoulder of the annular groove but its outer circumferential area slides against the inner face of the cylinder. The piston ring has a high rate of wear and there is little leakage of gas.
2. The collar of the piston ring reaches the stop face of the projecting shoulder of the annular groove. Wear of the piston ring against the inner face of the cylinder is reduced and the gas leakage is about the same as before.
3. An annular gap forms between the outer circumferential area of the piston ring and the inner face of the cylinder. There is relatively little wear and gas leakage is greater than before. The ring wear which occurred since phase No. 2 and possible further ring wear result from slight transverse piston movements in the cylinder. The better the piston and piston rod are guided, the less wear will occur. When the sealing annular gap between the piston ring and the inner face of the cylinder has become too large, the piston ring must be replaced.
Because of the angular or T-shape of the cross-section of the piston ring, its production is comparatively expensive. Because with the known piston rings the part extending from the projecting collar towards the inner face of the cylinder is bounded by two radial areas parallel with one another, which are guided between parallel areas on the projecting shoulder or shoulders of the annular groove, operation of the compressor in accordance with Phase 3 can lead to so-called fluttering of the rings. This is an uncontrolled motion of the piston ring within its annular groove. One attempts to avoid such fluttering either with gas pressure acting on the inner circumferential area of the piston ring or with tension springs resting against this area. This is, however, only partially successful and the use of tension springs increases costs.